The invention resides in a process for destroying chlorinated aromatic compounds of an exhaust gas which also contains sulfur dioxide, nitrogen oxides and dust.
In a number of high temperature processes such as waste incineration, the melting of ores or scrap metal and during sintering polychlorinated, aromatic hydrocarbons such as dibenzodioxines (PCDD), chlorophenols, and chlorobenzenes are formed and emitted into the atmosphere if no means are provided for retaining them. The polychlorinated aromatic hydrocarbons could be gaseous or could be present as particles.
Gaseous polychlorinated aromatic hydrocarbons can be separated from the exhaust gas at low temperatures by adsorption processes.
However, in such an adsorption process, the relatively small mass flows of the polychlorinated aromatic hydrocarbons to be separated require large amounts of adsorbents which subsequently need to be incinerated or deposited underground. It would be better if the polychlorinated aromatic hydrocarbons could be destroyed in the exhaust gas flow so that no secondary waste would be generated.
From DE 38 41 858 C1, it is known that gaseous dioxines and furans can be oxidatively destroyed at higher temperatures by adding hydrogen peroxide to the exhaust gas.
It is also known that various contaminants in the exhaust gas can be destroyed by electron irradiation. Instead of electron irradiation, a number of other types of radiation such as ultraviolet, microwave and gamma radiation have been proposed.
Because of the generally unknown compound spectrum in technical waste gases such irradiation may well cause unforeseeable chemical reactions. The effect of the irradiation therefore depends to a great extent on the composition of the exhaust gases. It cannot be predicted whether a particular contaminant is destroyed by the irradiation.
Another problem in the use of such a process resides in the fact that, with higher temperatures, gaseous PCDD and PCDF can be newly formed from particle-bound carbon.
The new formation of such gaseous PCDD can even exceed the expected decomposition so that the treatment increases the amount of gaseous contaminants in the exhaust gas.
In addition to gaseous PCDD and PCDF, also PCDD and PCDF-containing aerosols can be formed.
Because of the greatly different vapor pressures of the various PCDD and PCDF, the formation of new contaminants in aerosols can also occur at relatively low temperatures. Particle bound PCDD and PCDF, however, cannot be destroyed by irradiation of the exhaust gases with the types of radiation referred to above.
EP 0 416 631 A1 discloses a method for the removal of contaminants from the exhaust gas of a waste incineration plant. In this method, the exhaust gas is conducted at 150.degree. to 300.degree. C. into a reactor in which a spray cloud of an alkaline material is generated and, at the same time, the exhaust gas is exposed in the reactor preferably to electron radiation. It is said in the patent publication that with this method chlorinated organic compounds can be destroyed together with other contaminants. In this method, the gas leaving the reactor is conducted through a dust collector which removes the reaction products of the alkaline material.
However, no dust separator is arranged ahead of the reactor so that all the particles contained in the exhaust gas enter the reactor. Under these circumstances, it is very likely that PCDD and PCDF are newly formed at the reactor exit so that no effective gas clean-up can be achieved. Furthermore, a large part of the radiation energy is consumed for the conversion of the nitrogen oxides and the sulfuric oxides so that the method becomes ineffective with regard to the organic chlorine compounds.
It is the object of the present invention to avoid these disadvantages. With the method according to the invention, the irradiation should be effective almost only for the destruction of the chlorinated aromatic compounds while renewed formation of these compounds in the form of gas as well as in connection with particles is effectively reduced.